The controlled heating of reaction vessels is often carried out using solid block heaters which are heated and cooled by various methods. Current solid block heaters are heated by electrical elements or thermoelectric devices inter alia. Other reaction vessels may be heated by halogen bulb/turbulent air arrangements. The vessels may be cooled by thermoelectric devices, compressor refrigerator technologies, forced air or cooling fluids. The reaction vessels fit into the block heater with a variety of levels of snugness. Thus, the thermal contact between the block heater and the reaction vessel varies from one design of heater to another. In reactions requiring multiple temperature stages, the temperature of the block heater can be adjusted using a programmable controller for example to allow thermal cycling to be carried out using the heaters.
This type of heater arrangement is particularly useful for reactions requiring thermal cycling, such as DNA amplification methods like the Polymerase Chain Reaction (PCR). PCR is a procedure for generating large quantities of a particular DNA sequence and is based upon DNA's characteristics of base pairing and precise copying of complementary DNA strands. Typical PCR involves a cycling process of three basic steps.
Denaturation: A mixture containing the PCR reagents (including the DNA to be copied, the individual nucleotide bases (A,T,G,C), suitable primers and polymerase enzyme) are heated to a predetermined temperature to separate the two strands of the target DNA.
Annealing: The mixture is then cooled to another predetermined temperature and the primers locate their complementary sequences on the DNA strands and bind to them.
Extension: The mixture is heated again to a further predetermined temperature. The polymerase enzyme (acting as a catalyst) joins the individual nucleotide bases to the end of the primer to form a new strand of DNA which is complementary to the sequence of the target DNA, the two strands being bound together.
A disadvantage of the known block heaters arises from the lag time required to allow the heating block to heat and cool to the temperatures required by the reaction. Thus, the time to complete each reaction cycle is partially determined by the thermal dynamics of the heater in addition to the rate of the reaction. For reactions involving numerous cycles and multiple temperature stages, this lag time significantly affects the time taken to complete the reaction. Thermal cyclers based on such block heaters typically take around 2 hours to complete 30 reaction cycles.
For many applications of the PCR technique it is desirable to complete the sequence of cycles in the minimum possible time. In particular for example where respiratory air or fluids or foods for human and animal stock consumption are suspected of contamination rapid diagnostic methods may save considerable money if not health, even lives.
An alternative thermal cycler contains a number of capillary reaction tubes which are suspended in air. The heating and cooling of the reaction tubes is effected using a halogen lamp and turbulent air from a fan. The thermal dynamics of this system represent a considerable improvement over the traditional block heater design because heated and cooled air is passed across the reaction tubes and the required temperatures are achieved quite rapidly, the fan providing a homogeneous thermal environment and forced cooling. Using this apparatus 30 reaction cycles can be completed in about 15 minutes.
A disadvantage of this thermal cycler is that air cooling and heating are not readily suitable in apparatus which is required to provide different thermal cycling conditions to multiple reactions at the same time, and is certainly not mobile or portable.